Colloidal semiconductor quantum dots (QDs), with the radii around several nanometers, have unique photoluminescence (PL) properties due to the quantum confinement effect of the charge carriers. QDs possess higher PL quantum efficiency, tunable luminescence depending only on their size, wide continuous absorption, narrower PL band, and higher photo stability over conventional organic fluorescent dyes. Since hydrophilic QDs were first used as fluorescence probes in cellular labeling in 1998, QDs have attracted widespread attention from the fields of biology and medicine and achieved remarkable progress in biomedical applications. Along with biomedical applications, sensing schemes based on fluorescence resonant energy transfer (FRET) or photo induced electron transfer (PET) have also been developed for detecting small molecules, and for tracing bio recognition events or bio catalytic transformations. Chen and Rosenzweig reported the first practical use of CdS QDs as chemical sensors to determine Zn (II) and Cu (II) ions in aqueous media. Recently, great attention has been paid to the applications of QDs as metal ion probes because of the environmental and biological importance of metal ions. Depending on the nature of QDs as well as surface coatings, a number of QDs-based probes were developed for transition metal ions, including Hg2+, Cu2+, Ag+, and Pb2+. Nevertheless, very little attention has been paid to the fluorescence behavior of QDs in the presence of various chemical species mainly anionic pollutants.
An article titled, “Eu3+-doped water-soluble CdTe quantum dots prepared by one-pot approach” by B. R. C. Vale, J. C. L. Souza, J. L. Ferrari, M. A. Schiavon reports the Eu3+-doped MPA-CdTe, wherein a red-shift was observed for both absorption and emission spectra, with an increase of PL intensity.
An article titled, “Detection of Hg2+ and F− ions by using fluorescence switching of quantum dots in an Au-cluster-CdTe QD nanocomposite” by Paramanik B, Bhattacharyya S, Patra A in Chemistry, 2013 May 3; 19(19):5980-7 reports a single probe of an Au nanocluster-CdTe quantum dots nanocomposite has been developed by using tripeptide-capped CdTe quantum dots (QD) and bovine serum albumin (BSA) protein-conjugated Au25 nanocluster (NC) for detection of both Hg(2+) ion and F(−) ion. A significant signal off (74% PL quenching at 553 nm) phenomenon of this nanocomposite is observed in presence of 6.56×10(−7) M Hg(2+) ion, due to salt-induced aggregation. However, a dramatic PL enhancement (128%) of the Au-NC-CdTe QD nanocomposite is observed in presence of 8.47×10(−7) M F(−) anion. The calculated limit of detections (LOD) of Hg (2+) ion concentration and F(−) ion concentration are found to be 9 and 117 nM, respectively.
An article titled, “Highly fluorescent fluoride-responsive hydrogels embedded with CdTe quantum dots” by Juan Zhou , Haibing Li in ACS Applied Materials & Interfaces January 2012; 4(2):721-4 reports functionalized CdTe quantum dots (QDs) synthesized via ion exchange that demonstrated a selective response toward fluoride in aqueous solutions based on a rapid sol-gel transition that was visible to the naked eye. The fluoride-induced hydrogel exhibited excellent fluorescent performance because of the incorporation of QDs. As a result, this highly fluorescent fluoride-induced hydrogel may pave a new way to sense fluoride using a visible sol-gel transition.
An article titled, “A new nanoprobe based on FRET between functional quantum dots and gold nanoparticles for fluoride anion and its applications for biological imaging” by Xue M, Wang X, Duan L, Gao W, Ji L, Tang B. in Biosens Bioelectron, 2012 June-July; 36 (1):168-73 reports a new nanoprobe that was designed for the fluorescence imaging of fluoride anion (F(−)) in living cells with high sensitivity and selectivity. The design is based on the fluorescence resonance energy transfer (FRET) between CdTe quantum dots (CdTe QDs) and gold nanoparticles (AuNPs) through the formation of cyclic esters between phenylborinic acid and diol.
An article titled, “Preparation of europium-quantum dots and europium-fluorescein composite nanoparticles available for ratiometric luminescent detection of metal ions” by Dong H, Liu Y, Wang D, Zhang W, Ye Z, Wang G, Yuan J. Nanotechnology, 2010 Oct. 1; 21(39):395504 reports two types of europium-containing dual-luminophore silica nanoparticles, silica-encapsulated CdTe quantum dots (CdTe QDs)-BHHCT-Eu(3+) complex nanoparticles and BHHCT-Eu(3+) surface-bound silica-encapsulated fluorescein isothiocyanate (FITC) nanoparticles (BHHCT: 4,4′-bis(1″,1″,1″,2″,2″,3″,3″-heptafluoro-4″,6″-hexanedion-6″-yl)chlorosulfo-o-terphenyl), were successfully prepared using a water-in-oil (W/O) reverse microemulsion method. The results of transmission electron microscopy and luminescence spectroscopy characterizations indicate that the two types of nanoparticles are all monodisperse, spherical and uniform in size (approximately 50 nm in diameter), and have well-resolved and stable dual luminescence emission properties. The CdTe QDs-BHHCT-Eu(3+) nanoparticles can be excited at 365 nm to give dual-emission peaks at 535 and 610 nm, and the FITC-BHHCT-Eu(3+) nanoparticles can be excited at 335 nm to give dual-emission peaks at 515 and 610 nm. The luminescence response investigations of the nanoparticles to different metal ions indicate that the new nanoparticles can be used as ratiometric luminescent sensing probes for the selective detection of Cu(2+) and Fe(2+) ions, respectively.
Owing to the sustained interest of selective chemosensor in environmental monitoring, the design of sensing devices based on QDs is a topic of great interest.
On the other hand, Fluoride anion has unique chemical and physiological properties, and it is also associated with nerve gases, analysis of drinking water, and the refinement of uranium. Therefore, it is necessary for environmental or biological systems to develop the effective fluorescent sensors selectively for fluoride ion.
Thus, it may be summarized that the prior art reports doping of Eu3+ in CdTe QDs but there is no doping in the present case. Inventors have introduced Eu3+ in solution which is covalently attached with carboxylic acid groups present in the surface of CdTe QDs results PL quenching of bare MPA capped CdTe QDs. Further, some inventors have used Au nanoclusters on CdTe QDs as a base system whereas the present invention uses simple CdTe QDs as a base PL sensor. Thirdly, no record is available that is applicable for HF fume detection.
Accordingly, keeping in view the drawbacks of the hitherto reported prior art, the inventors of the present invention realized that there exists a dire need to provide a sensitive, rapid (as response time is one of the important parameters for sensor application and quick response is an essential requirement to take further action) and selective process for the detection of fluoride ions in solution as well as gaseous form.